Homologous recombination occurs in almost all organisms. Homologous recombination can lead to the occurrence of certain forms of cancer, such as retinoblastoma. The mechanism of this recombination is not completely understood. Much greater detail regarding this mechanisms and its control is necessary before the potential of gene replacement therapy to treat diseases such as Lesch-Nyhan syndrome is fully realized. The study of special sites called recombination hotspots has provided insight into this mechanism. These hotspots function to produce a localized stimulation of recombination. One such hotspot, HOT1, is a part of the ribosomal DNA (rDNA) repeat unit of yeast. HOT1 stimulates mitotic recombination when inserted at novel locations in the yeast genome. The sequences required for HOT1 activity correspond closely to the sequences necessary for efficient transcription of the precursor 35S rRNA. Thus it appears that there is an association between recombination and transcription. Since HOT1 is a part of the rDNA repeat unit, it may play a role in maintaining sequence homogeneity among the rDNA repeats. The long-term objective of the work proposed is to gain further understanding of the mechanism of recombination in yeast. The current focus of this work os to study the mechanism by which HOT1 stimulates recombination and to characterize the function of this hotspot in rDNA recombination. Genetic experiments have identified trans-acting mutations that affect the activity of HOT1 or rDNA recombination. A specific aim of this work is to study the roles of these factors in recombination. This will involve characterization of the effects of these mutations on different forms of genetic recombination and molecular cloning and characterization of the genes identified by these mutations. Another specific aim is to identify the cis-acting sequences required for HOT1 activity. The mutations produced in these studies will be used to further analyze the relationship between transcription and recombination.